fix: .terraform-plan job

Closes #95 (closed)

In expression

(terraform plan $TF_PLAN_ARGS -out=${TF_DATA_DIR}/${CI_COMMIT_REF_SLUG}-${DEPLOYMENT_ENVIRONMENT}.tfplan -detailed-exitcode && exit_code=$?) || exit_code=$?

the variable exit_code is set in a sub-shell ( ( ... ) ). Because of that, if actual exit code is zero, the following script can't use it. Example:

bash-5.2$ (terraform plan $TF_PLAN_ARGS -out=123.tfplan -detailed-exitcode && exit_code=$?)

bash-5.2$ echo $exit_code

bash-5.2$

A subshell inherits the environment of the parent shell, but its exit status and environment changes (like setting variables) do not persist back to the parent shell.